The flags indicate the countries that a campaign is targeted at. If you see the Stars and Stripes, the Union Jack and the Tricolor, you know the campaign is targeted at the US, UK and France. (Hover over the flags to see which country they represent).

You can run campaigns that aren’t targeted at your country, but you may earn less, as a campaign targeted at, say, Spain and Portugal will only pay out for views that occur in Spain and Portugal.